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Citation: Manea, E.E.; Bumbac, C.;
Dinu, L.R.; Bumbac, M.; Nicolescu,
C.M. Composting as a Sustainable
Solution for Organic Solid Waste
Management: Current Practices and
Potential Improvements. Sustainability
2024,16, 6329. https://doi.org/
10.3390/su16156329
Academic Editors: Zakaria Solaiman,
Shamim Mia and Md. Abdul Kader
Received: 7 June 2024
Revised: 9 July 2024
Accepted: 22 July 2024
Published: 24 July 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
sustainability
Review
Composting as a Sustainable Solution for Organic Solid Waste
Management: Current Practices and Potential Improvements
Elena Elisabeta Manea 1, Costel Bumbac 1, * , Laurentiu Razvan Dinu 1, Marius Bumbac 2
and Cristina Mihaela Nicolescu 3, *
1
National Research and Development Institute for Industrial Ecology—ECOIND, 57–73 Drumul Podu Dambovitei,
District 6, 060652 Bucharest, Romania; elena.manea@incdecoind.ro (E.E.M.)
2Faculty of Science and Arts, Valahia University of Targoviste, 13 Aleea Sinaia, 130004 Targoviste, Romania;
marius.bumbac@valahia.ro
3Institute of Multidisciplinary Research for Science and Technology, Valahia University of Targoviste,
13 Aleea Sinaia, 130004 Targoviste, Romania
*Correspondence: costel.bumbac@incdecoind.ro (C.B.); cristina.nicolescu@valahia.ro (C.M.N.)
Abstract: With increases in global population and urbanization, the production of Municipal Solid
Waste (MSW) is growing rapidly, thus contributing to social and environmental concerns for sus-
tainable waste management. This study addresses the research gap in optimizing composting,
hypothesizing that integrating best practices and recent innovations can enhance the efficiency of the
process. Data were collected through a systematic review of existing literature using Google Scholar
and Scopus databases. The review provides an overview of municipal organic waste composting,
outlining its processes, benefits, and challenges with the aim of identifying key area of further im-
provement and possibilities of adopting recent technological innovations. The analysis emphasized
that technological advances in composting, as microbial inoculants or in-vessel composting have
greatly improved the efficiency and quality of the resulting compost. However, several challenges
remain, including managing contaminants such as heavy metals and microplastics, ensuring the
compost quality and safety and addressing socioeconomic barriers that prevent widespread adoption.
Moreover, process optimization, environmental and economic evaluation, as well as political and
public involvement are essential to unlock the whole potential of composting systems.
Keywords: nutrient recycling; organic waste composting; municipal waste management; sustainable
resource utilization; soil amendment
1. Introduction
Population growth, urbanization, and economic advances have directly accelerated
and contributed to increased generation of municipal solid waste (MSW) [
1
,
2
]. Such surging
waste production leads to significant concerns and challenges for sustainable waste manage-
ment systems development, environment conservation, and resource recovery [
3
]. Among
the different constituents of MSW, the organic fraction, primarily food waste, garden
wastes, and other biodegradable materials accounts for a large fraction (40–70%), particu-
larly higher in developing countries [
2
]. This organic fraction represents a challenge and
an opportunity for innovative waste treatment solutions development [
4
,
5
]. The negative
environmental consequences of traditional waste disposal methods have aroused growing
criticisms, landfilling and incineration leading to greenhouse gas (GHG) emissions, leachate
production, and valuable organic carbon loss [
6
,
7
]. Landfilling biodegradable organic waste
generates significant amounts of methane (CH
4
) and ammonia (NH
3
), powerful GHGs
that contribute to climate change [
8
–
10
]. On the other hand, incineration generates CO
2
emissions together with other potentially toxic contaminants such as dioxins [
2
]. In this
context, the need for sustainable waste management practices that not only address envi-
ronmental hazard mitigation but also actively promote resource recovery is more pressing
Sustainability 2024,16, 6329. https://doi.org/10.3390/su16156329 https://www.mdpi.com/journal/sustainability
Sustainability 2024,16, 6329 2 of 25
than ever. Composting involves a series of aerobic transformations of organics through
microorganisms’ activity with the production of a stable product known as compost [
11
–
13
].
Composting offers numerous environmental advantages such as reducing the volume of
waste landfilled, reducing pestilential odors associated with the anaerobic degradation of
organic waste, reducing methane emissions, and transforming organic wastes into valuable
nutrient-enriched soil amendments [
14
,
15
]. Moreover, using compost as a soil amendment
has the potential to enhance soil fertility, increase carbon sequestration, and improve soil
structure, physical, chemical, and biological properties [
16
]. Thus, the process of compost-
ing perfectly integrates into the desired circular economy context by promoting nutrient
recycling and diminishing the demand for synthetic fertilizers supporting, at the same time,
sustainable agriculture practices and long-term food security [17,18].
Improvements in composting technology and practices have increased its efficacy
and relevance. Techniques such as in-vessel composting, vermicomposting, and the use
of microbial inoculants have augmented the composting efficiency and the quality of the
compost. Moreover, the merger of composting with different waste administration solu-
tions like anaerobic digestion has created supplementary alternatives for useful resource
restoration and power generation [19].
Beyond having multiple environmental and economic benefits, the widespread adop-
tion of the composting process as a mainstream waste management solution faces some
technical challenges and socio-economic barriers. Most concerns are related to the presence
of contaminants like heavy metals and microplastics in feedstock and final compost and
the predictability of quality and safety of compost produced [
11
]. To alleviate these issues
well-rounded management practices, regulatory frameworks and policy implementation
plans are necessary.
The main objective of this paper is to provide a comprehensive review of organic solid
waste composting in terms of processes, benefits, and challenges, and it also includes a
synthesis of current knowledge and identified research gaps, trying to provide new insights
into modern composting technology and practices and future research opportunities to
strengthen the composting science and practices and to foster sustainability and resilience
of waste management.
2. Review Methodology
The study was based on high-impact scientific papers identified in international
databases (such as Science Direct, Web of Science, Scopus, IEEE Xplore, and Google Scholar),
with an accent on recent studies. Of the multiple results, 173 research papers were included
in the review, responding to the main research area of interest. Of these papers, 38 were
published in 2023, 39 in 2022, and 25 in 2021 showing the high interest researchers have
in composting municipal solid waste. The research studied was categorized in the main
review chapters, focusing on municipal waste composition, composting process, nutrient
transformation, the fate of contaminants during composting, benefits and challenges, case
studies, and future directions.
3. Composition of Municipal Organic Waste
MSW contains a wide variety of materials, primarily originating from residential house-
holds, commercial, and garden sources with biodegradable organic waste as a predominant
fraction. This fraction includes kitchen waste, yard trimmings, and other biodegradable
materials susceptible to recuperative treatment through anaerobic digestion, composting,
or a combination of both technologies thus contributing to the goals of the European Cir-
cular Economy Package and reducing emissions associated with landfill disposal [
20
]. In
particular, there are several types of organic wastes, which can be classified by constitution
in paper and cardboard, textiles, biodegradable plastics, biodegradable kitchen waste, and
garden trimmings. For example, paper waste can be composed of receipts and newspapers,
whereas cardboard waste may consist of egg cartons or contaminated pizza boxes, as exam-
ples. Textiles can be represented by obsolete clothing items made of natural fibers such as
Sustainability 2024,16, 6329 3 of 25
cotton, hemp, silk, or linen. Compostable, biodegradable plastics present in the organic
fraction of MSW may be represented, for instance by starch or cellulose-based plastics,
polyhydroxyalkanoates, polybutylene succinate, or others. Kitchen waste fraction refers
to the organic waste generated from food preparation and consumption in households,
restaurants, and other food service establishments and explicitly consists of items like food
scraps (vegetable and fruit peelings, leftovers and spoiled food, eggshells and husks, coffee
grounds and tea bags, paper towel and napkins) [
21
]. Yard trimmings are the organic waste
materials resulting from gardening and landscaping and consist of grass clippings, leaves,
trimmings, garden debris, weeds, woodchip bark, etc.
The main challenges when using biodegradable organic fractions of municipal solid
waste as feedstock for composting are represented by the need to identify the proper
balance of components to ensure the optimum C: N ratio and the potential presence of
contaminant/microcontaminants [2].
Organic waste streams present substantial variation in macronutrient concentrations,
such as nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), Magnesium (Mg), and
Sulfur (S) as well as, in many cases, micronutrients such as boron, copper, manganese,
and zinc (Table 1) [
22
–
24
]. Their concentrations and availability within the final compost
are influenced by factors such as the initial organic waste composition, animal farming
strategies, waste treatment processes, and additives. Sources of waste such as animal
manure, biogas digestates, and composts can be extremely rich in macronutrients like N, P,
and K, in the desired concentration range for plant growth. For example, animal manures
and biogas digestates can have high variability in their nutrient concentrations caused by
variations in feed composition and animal species [25].
Table 1. Composition and role of macronutrients.
Nutrient Role as Macronutrient Typical Range (g/kg) References
Nitrogen (N)
Essential for the growth of microorganisms, helps in
the decomposition 10–20 [26]
Phosphorus (P) Helps plants growth 2–5 [27,28]
Potassium (K) Enhances plant health and disease resistance 3–15 [27,29]
Calcium (Ca) Enhances the disease resistance and plant growth 4–6 [30,31]
Magnesium (Mg) Essential nutrient for plants <0.1 [30,31]
Sulfur (S) Microbial activity and some functions of plants 1–3 [32,33]
Nitrogen (N) has the highest concentration in compost (10–20 g/kg) compared to other
nutrients, also playing a significant role in plant growth and microbial activity.
All listed nutrients (N, P, K, Ca, Mg, S) are essential for various plant functions
including growth, disease resistance, plant health, and microbial activity.
The variability of nutrient concentrations leads to the necessity of knowing precisely
what the actual concentrations are, both to supply crops with optimal amounts of nutrients
and to minimize environmental problems due to over-fertilization [
34
,
35
]. The nutrient
content of organic wastes and composts can be determined by classical chemical analyses,
or by alternative methods such as X-ray fluorescence (XRF) spectrometry and near-infrared
spectrometry (NIRS) [
25
,
35
]. These techniques have proven, due to their speed, non-
destructive nature, and potential for automation, promising for assessing the bioavailability
of both macronutrients and micro-nutrients; nevertheless, their accuracy varies because
of the heterogeneity of the samples [
36
]. To meet this challenge, standardized sampling
and sample preparation protocols backed by the use of appropriate calibration methods
with well-characterized reference materials, and regular complementary analysis using
traditional chemical methods should be implemented in large-scale composting facilities to
reach a more comprehensive image of the nutrient content. The organic waste treatment
Sustainability 2024,16, 6329 4 of 25
solution applied, either composting or vermicomposting, can influence both the nutritional
content and the availability of compost [34].
Though composting is one of the most valuable methods for converting organic waste
into a nutrient-rich soil amendment, it should be mentioned that the presence of potentially
toxic substances (PTS) in MSW is a specific issue of concern [37,38]. The most challenging
PTS is potentially present in the organic fraction of municipal solid waste (OFMSW) con-
cerning the occurrence are heavy metals, micro- and nano-plastics (MPs and NPs) including
those coming from bioplastics (MBPs and NBPs), persistent organic pollutants (POPs),
pharmaceuticals and personal care products. This points out OFMSW’s composition com-
plexity and the necessity of adopting proper collection strategies and measures to mitigate
the occurrence of PTS in the composting feedstock [
14
,
22
–
24
]. Characteristics of PTS can
vary widely during composting depending on factors such as composting temperature,
moisture content, and feedstock composition.
4. Composting Process Overview
The composting process of municipal organic waste involves several steps designed to
ensure mainly proper feedstock management (collection, sorting, screening, pre-processing,
and mixing) and optimum biochemical conversion (composting and maturation) towards a
stable final product (Figure 1).
Sustainability 2024, 16, x FOR PEER REVIEW 4 of 26
sampling and sample preparation protocols backed by the use of appropriate calibration
methods with well-characterized reference materials, and regular complementary analy-
sis using traditional chemical methods should be implemented in large-scale composting
facilities to reach a more comprehensive image of the nutrient content. The organic waste
treatment solution applied, either composting or vermicomposting, can influence both the
nutritional content and the availability of compost [34].
Though composting is one of the most valuable methods for converting organic
waste into a nutrient-rich soil amendment, it should be mentioned that the presence of
potentially toxic substances (PTS) in MSW is a specific issue of concern [37,38]. The most
challenging PTS is potentially present in the organic fraction of municipal solid waste
(OFMSW) concerning the occurrence are heavy metals, micro- and nano-plastics (MPs and
NPs) including those coming from bioplastics (MBPs and NBPs), persistent organic pol-
lutants (POPs), pharmaceuticals and personal care products. This points out OFMSW’s
composition complexity and the necessity of adopting proper collection strategies and
measures to mitigate the occurrence of PTS in the composting feedstock [14,22–24]. Char-
acteristics of PTS can vary widely during composting depending on factors such as com-
posting temperature, moisture content, and feedstock composition.
4. Composting Process Overview
The composting process of municipal organic waste involves several steps designed
to ensure mainly proper feedstock management (collection, sorting, screening, pre-pro-
cessing, and mixing) and optimum biochemical conversion (composting and maturation)
towards a stable final product (Figure 1).
Figure 1. Composting process overview.
4.1. Collecting Organic Materials
The first step involves collecting organic materials, such as food scraps, yard trim-
mings, and agricultural wastes, from residential households and business communities.
One of the definite advantages of source separation over mechanical sorting in integrated
waste management systems is the fact that it lowers the potential risk of contamination
with plastics, metals, or personal care products-related contaminants. The result is a
cleaner main feedstock that requires less processing and allows a more balanced carbon-
to-nitrogen ratio which translates further into optimum microbial activity, faster decom-
position rates, a lower potential for ammonia emissions, and a final compost with fewer
contaminants of concern, and a more desirable nutrient profile [39].
4.2. Sorting and Screening
Sorting and screening procedures are designed to ensure a clean feedstock by remov-
ing inorganic contaminants such as plastics and metals. Initially, waste is segregated into
various categories using mechanical systems that may include shredders, trommels, and
magnetic separators to remove metals and other non-organic materials [40]. Additionally,
optical sensors, such as infrared and color sensors, are used to further sort waste paper,
plastics, and other recyclables, enhancing the efficiency of the sorting process [40]. More-
over, the latest developments in the field include advanced robotic systems using artificial
intelligence-based algorithms which are being developed to automate waste sorting, for
robust detection and manipulation of waste items, aiming to reduce human labor and
improve sorting accuracy [41].
Figure 1. Composting process overview.
4.1. Collecting Organic Materials
The first step involves collecting organic materials, such as food scraps, yard trim-
mings, and agricultural wastes, from residential households and business communities.
One of the definite advantages of source separation over mechanical sorting in integrated
waste management systems is the fact that it lowers the potential risk of contamination
with plastics, metals, or personal care products-related contaminants. The result is a cleaner
main feedstock that requires less processing and allows a more balanced carbon-to-nitrogen
ratio which translates further into optimum microbial activity, faster decomposition rates, a
lower potential for ammonia emissions, and a final compost with fewer contaminants of
concern, and a more desirable nutrient profile [39].
4.2. Sorting and Screening
Sorting and screening procedures are designed to ensure a clean feedstock by remov-
ing inorganic contaminants such as plastics and metals. Initially, waste is segregated into
various categories using mechanical systems that may include shredders, trommels, and
magnetic separators to remove metals and other non-organic materials [
40
]. Additionally,
optical sensors, such as infrared and color sensors, are used to further sort waste paper,
plastics, and other recyclables, enhancing the efficiency of the sorting process [
40
]. More-
over, the latest developments in the field include advanced robotic systems using artificial
intelligence-based algorithms which are being developed to automate waste sorting, for
robust detection and manipulation of waste items, aiming to reduce human labor and
improve sorting accuracy [41].
4.3. Pre-Processing of Organic Wastes and Bulking Agents
Pre-processing of organic wastes and bulking agents involves operations of shredding
or crushing designed to reduce materials size, increase specific surface area, and increase the
Sustainability 2024,16, 6329 5 of 25
number of spaces inside the compost pile that may facilitate optimal gas diffusion, mainly
oxygen (O
2
) that is essential for the activity of thermophilic microorganisms responsible
for breaking down the C-rich organic materials.
4.4. Feedstock Management and Mixing
Proper feedstock management and mixing is the cornerstone of a C/N balanced
composting process as it is directly responsible for microbial growth, with an optimal range
suggested to be between 25 and 35% [
42
–
44
]. In general, carbon-rich “brown” materials,
such as dry leaves (30–80:1), straw (40–100:1), sawdust (200–500:1) or paper (150–200:1) are
mixed with nitrogen-rich “green” materials, such as grass clippings (15–25:1) food scraps
(15–25:1), manure (5–25:1), coffee grounds (20:1) to achieve the optimum balance [
2
,
7
,
37
].
Adjusting the C/N ratio is essential for promoting microbial activity and ensuring efficient
composting [
44
]. A low C/N ratio can lead to higher emissions of GHGs and odorous
substances like ammonia due to nitrogen mineralization, while a high C/N ratio can lead
to severe issues such as low biological activity, lower degradation rates, insufficient heating
of the composting pile, and longer composting periods [39,42,43,45].
4.5. Composting
A controlled mixture of organic matter is prepared and formed into windrows (aerobic
piles) outdoors, or placed in enclosed vessels for in-vessel composting. Inside the compost
pile, microorganisms including bacteria and fungi use oxygen to decompose the organic
matter. This decomposition process results in heat generation leading to a thermophilic
phase with temperatures ranging between 45
−
55
◦
C with peaks reaching sometimes
up to approximately 80
◦
C [
39
]. The high temperature destroys pathogens and weed
seeds ensuring a sanitized final product. During the composting process, maintaining
optimal moisture content (around 50–60%) is essential for supporting microbial activity
and maintaining the physical properties of the pile [
45
]. This is usually performed during
regular turning of the compost pile which is crucial also for ensuring proper aeration [46].
4.6. Factors Influencing Composting Effectiveness
Excessive moisture can lead to anaerobic conditions, while too little moisture can in-
hibit microbial growth and activity and hampers organic matter mineralization as described
in a study focused on grape marc and stalks composting [46,47].
Microorganisms have an essential role in municipal organic waste composting by
contributing to nutrient mineralization, immobilization, and organic matter degradation.
Different raw materials used as feedstock in composting have varied natural microflora,
which plays the role of keystone taxa to steady active composting and maturation. More-
over, the addition of microbial inoculants has been identified as a beneficial strategy to
enhance the biotransformation of organic materials during composting. The important role
played by Bacillus and Thermus genera in the thermophilic stage is often underlined, with
their thermostable enzymes such as proteases, cellulases, and lignin-modifying enzymes
that are indispensable during the degradation of organic matter [
48
–
50
]. Research confirms
that supplementation of microbial agents by thermophilic aerobic bacteria significantly
improves organic waste degradation efficiency [
48
–
50
], proving that the selection of specific
microbial consortia is crucial for the success of composting. In addition, the exploitation
and application of valuable yeasts and filamentous fungi that possess high biotechnological
potential such as Pichia kudriavzevii and Aspergillus spp. can act as starter cultures to achieve
faster composting [
51
]. These indicate the importance of selecting specific microbial con-
sortia for improved composting performances [
52
,
53
]. During composting, the diversity,
composition, and function of microbial communities are significantly influenced by ambi-
ent parameters and physicochemical characteristics of feedstock, such as temperature and
moisture content as well as total organic carbon, nitrogen, and phosphorus which govern
the activity of microbial species, highlighting the importance of microbial interactions
in the composting process [
17
,
42
,
43
,
54
]. Moreover, the presence of bioplastics has been
Sustainability 2024,16, 6329 6 of 25
reported to influence the composition and activity of the microbial community during
composting, particularly during the aerobic phase [
24
,
50
]. Microbial-mediated nitrogen
and sulfur cycles are also integral to the decomposition of MSW, affecting carbon cycling
and emissions of GHGs [52,53].
4.7. Maturation
The maturation stage of municipal organic waste composting is crucial to ensure the
stability and quality of the final compost product. In this stage, the temperature drops and
the biological processes that govern the process are represented by the reduction of easily
biodegradable carbon fractions, the increase in the concentration of humic substances,
and the nitrification, respectively, the transformation of the remaining ammonium into
nitrate [
55
–
57
]. Thus, the indicators of maturation and stabilization of the final compost are
the high concentrations of humic substances and nitrates.
Recent research has indicated that the type of raw material used significantly affects
microbial dynamics and enzyme activity during maturation, with different types of waste
supporting different microbial communities [
58
]. For example, fungi tend to dominate in
fish sludge compost, while bacteria are more common in manure and municipal sludge
compost [59].
5. Nutrient Transformation and Fate of Contaminants during Composting
5.1. Nutrient Transformations
The transformation of biodegradable municipal organic waste into a nutrient-rich
compost involves going through a sequence of complex biological and physicochemical
processes influenced by various factors, including the composting method, the addition of
bulking agents, and the presence of microorganisms. Key nutrient transformations during
this process include the mineralization and mobilization of nitrogen (N), phosphorus (P),
potassium (K), calcium (Ca), magnesium (Mg), and sodium (Na). Studies have shown
that depending on the composting method chosen (among aerobic composting, anaerobic
composting, and co-composting), it significantly influences the total losses of carbon (C)
and nitrogen (N), the anaerobic composting method is indicated to have the lowest losses
of C and N during the composting process [55,56].
During the composting process, nitrogen compounds undergo several successive
transformations that involve both biological processes of ammonification, nitrification, and
denitrification, as well as physical-chemical processes of immobilization and loss through
leaching or volatilization. The ammonification process involves the enzymatic breakdown
of proteins and other nitrogen compounds into amino acids and subsequently into am-
monia (NH
3
) and ammonium ions (NH
4+
) [
57
]. Ammonification ensures the availability
of nitrogen, in the form of ammonium, for the subsequent processes of nitrification and
denitrification [
60
,
61
]. The intensity of the ammonification process depends on the type of
raw materials subjected to composting and the initial microbiota of the mixture [57,58,62].
Thus, the concentration of ammonium (NH4+) increases to a maximum level in the initial,
thermophilic phases of composting, followed by a gradual decrease through the transfor-
mation of the accumulated ammonium into nitrate as a result of the biological nitrification
processes [
58
–
60
]. Nitrification begins when the temperature of the composting mixture
drops below 40
◦
C, with the highest nitrate concentrations observed at the end of the
maturation phase [
59
,
61
]. The biological transformations of different forms of nitrogen,
during composting, are enzymatically mediated by enzymes such as nitrate reductase,
nitrite reductase, and urease [
58
]. The composting process generally results in an increase
in total nitrogen (Nt) and nitrate nitrogen (N-NO
3−
), while ammonia nitrogen (N-NH
4+
)
decreases [
54
]. However, significant attention must be paid to nitrogen losses that may
occur through ammonia volatilization, especially when high ammonia concentrations co-
incide with high pH values in the composting system [
14
,
62
,
63
]. In addition, prolonged
anaerobic conditions can inhibit nitrification, leading to a higher ammonium-to-nitrate ratio
and delayed compost maturation [
59
]. Also, the use of earthworms in vermicomposting
Sustainability 2024,16, 6329 7 of 25
can, through the specific enzyme system, additionally improve nitrogen transformation
processes, leading to higher concentrations of assimilable nitrogen in the final compost
compared to traditional composting methods [64,65].
In composting, Phosphorus (P) can be identified in various forms such as inorganic P,
organic P, water-soluble P, citric acid P, etc. Many researchers also reported that phosphorus
available form changes considerably during composting processes [
36
,
66
,
67
]. For example,
the availability of phosphorus will decrease during the thermophilic phase of compost-
ing but increase again during the compost maturation phase. Moreover, the composting
feedstock (organic waste) plays an important role in phosphorus form changes during
the processes. Studies have shown that poultry and pork manure have higher phosphor
contents in comparison with other kinds of organic waste [
67
]. Microbial activity in the
composting process plays an essential role in phosphorus transformation while the inocula-
tion with specific microbial consortia such as P. chrysosporium,T. viride, and
P. aeruginosa
has been found to boost enzymatic activity, thereby speeding up the composting process
and improving the nutritional value of compost [68].
The presence of certain bacteria groups is highly related to the release of inorganic
phosphate into the soil and bounded phosphorus forms, especially bacteria harboring the
phosphatase gene (phoD). The enzyme activities, such as alkaline phosphatase activity,
are closely related to the transformation of P fractions by promoting the conversion of
organic P into inorganic P, making it easily available to plants [
68
]. In particular, the specific
case of vermicomposting shows a significant increase in phosphorus (24.9–45.8%) and
potassium (24.9–45.8%) which is due to gut enzymes that help to release and mineralize
these elements [
64
]. Moreover, the addition of rock phosphate during composting can
further enhance the dissolution of mineral elements, contributing to the overall nutrient
content of the compost [69].
The evolution of potassium during the composting process of municipal waste is rela-
tively stable compared to other nutrients, such as nitrogen and phosphorus. This stability is
due to potassium’s non-volatile nature and its resistance to loss through biochemical or mi-
crobiological processes. Thus, as the organic matter decomposes, the potassium originally
present in the organic waste remains in the resulting compost in the form of soluble salts,
thus becoming easily available to plants [
52
,
69
]. As an example, the compost resulting from
municipal solid waste has been shown to significantly bind potassium to organic matter,
making it a viable alternative to traditional cattle manure for supplying potassium to crops
such as lowland rice [
70
]. Similarly, calcium and magnesium levels increase because of the
degradation processes occurring during composting as they are released from the organic
matter and become more concentrated in the compost due to the reduction in organic
mass through mineralization [
10
,
66
]. Sodium levels can also rise initially but may fluctuate
depending on the source of the waste and leaching processes. For the specific case of
vermicomposting, compared to traditional composting, researchers have reported a higher
increase in magnesium (12.2–63.8%) and sodium (30.2–40.5%) concentrations [
64
]. These
elements are important for improving soil structure and fertility.
The composting process is significantly influenced by the carbon dynamics. The
dynamics of carbon and humic substances are governed by the microbial degradation
processes of organic matter resulting in the release of carbon dioxide and heat concomi-
tant with the generation of humic substances including humic acids, fulvic acids, and
humans [
44
]. The concentration of humic substances increases as the composting process
progresses, including during the maturation phase. Thus, at the end of the composting
process, humic substances have a significant proportion of organic matter, thus marking
the stabilization of the compost and its readiness for further use in agriculture [44,46].
Humic acid (HA) undergoes substantial transformations throughout composting. It
has been reported that during the first stages of composting, approximately 50% of the
total concentration of HA is reduced, whereas the core HA remains relatively stable [
71
,
72
].
This transformation is characterized by a degradation of coating materials, such as polysac-
charides, peptides, and lipids, resulting in HA structures of higher aromaticity [
72
]. The
Sustainability 2024,16, 6329 8 of 25
humification process, which involves the formation of humic-like substances, is influenced
by the degradation rates of water-soluble carbohydrates and phenols, which serve as pre-
cursors to humification [
73
]. Other studies indicated that the humic acid content typically
increases to a peak around 110 days into the composting process, indicating a maturation
phase where the organic matter transforms into a more aromatic structure [
74
]. Detailed
analyses of spectroscopy revealed significant structural changes in HA as indicated by
the increase in aromatic and phenolic C-containing groups [
72
]. These transformations
collectively demonstrate the dynamic nature of carbon and humic substances during the
composting of municipal organic waste. The quantity and quality of humic substances (HS)
in compost are considered key indicators of compost maturity and chemical stability [
10
].
Mature compost usually has higher humus concentrations confirming the transformation
of organic matter and nitrogen during composting [10].
5.2. Fate of Contaminants
MSW composts contain heavy metal concentrations that are higher than background
soil levels. The most common heavy metals identified in MSW composts are zinc (Zn),
lead (Pb), copper (Cu), cadmium (Cd), and nickel (Ni) [
75
–
77
]. Their environmental
impact and bioavailability for plant uptake is a critical concern. Generally, the composting
process changes the speciation of heavy metals, which normally decreases the solubility
and bioavailability of heavy metals [
76
,
77
]. For example, the complexation of heavy metals
with organic matter occurring during aerobic composting plays a key role in the decrease
of heavy metals solubility and bioavailability [
76
,
77
]. Heavy metals, such as Pb and Cu,
have been reported to show a significant reduction in water-extractable fraction during
composting, thus indicating a shift towards more stable forms [
78
]. The mobility and,
implicitly, the bioavailability of heavy metals in composts are influenced by several factors,
such as pH, organic matter content, and humification degree. Thus, a higher pH level in
MSW compost leads to reduced heavy metals mobility. However, this correlation between
pH and metal mobility is not definitive and straightforward [
78
,
79
]. The stabilized organic
matter is another key factor that affects the heavy metals’ mobility, the more stable organic
matter is, the less mobility of heavy metals occurs [78,79].
Another critical environmental and technological concern is the occurrence and evolu-
tion of POPs. POPs include, among other substances, polychlorinated biphenyls (PCBs),
dioxins, and certain pesticides, which are initially present in the composting feedstock due
to their widespread use and environmental persistence. The capacity of the composting
process to fully degrade the persistent organic pollutants varies significantly depending on
the nature of the compounds. For instance, sodium linear dodecylbenzene sulfonate (LAS)
shows a high mineralization rate of 51%, while nonylphenol (NP) and glyphosate exhibit
intermediate rates of 29% and 24%, respectively. Fluoranthene, however, shows negligible
mineralization [
80
]. A significant portion of some pollutants becomes non-extractable
residues (NER). For example, 45% of NP and 37% of glyphosate are found as NER at the
end of the composting process. This indicates that these compounds are stabilized in the
compost matrix, reducing their bioavailability [
71
]. A study identified 121 volatile organic
compounds (VOCs) generated during the composting of municipal biowaste, including
highly toxic N-containing compounds. These emissions were influenced by the type of
waste and the composting conditions [81].
Microplastics persist and fragment during the composting of municipal organic waste,
with both conventional plastics and bioplastics contributing to their presence in com-
post [
82
,
83
]. For example, macroplastics such as expanded polystyrene (EPS), polypropy-
lene (PP), and polyethylene (PE) can release numerous MP particles due to mechanical
forces, oxidation, and biodegradation [
84
]. Additionally, the degradation of bioplastics in
composting environments varies, with some bioplastics showing significant degradation
while others persist. For instance, starch-based shopping bags (SBSB) and polylactic acid
(PLA) tableware showed different degradation rates, with complete degradation expected
in 1.6 years for SBSB and 7.2 years for PLA [
85
,
86
]. The presence of microplastics (MPs) in
Sustainability 2024,16, 6329 9 of 25
the composting process of organic waste can significantly influence various aspects of com-
post quality, microbial communities, and the overall composting dynamics. Microplastics
can alter the humification process during composting. For instance, the addition of different
types of MPs such as PE, polyvinyl chloride (PVC), and polyhydroxyalkanoates (PHA)
have been shown to reduce the humic acid to fulvic acid ratio, indicating a lower degree of
humification compared to control treatments without MPs [
87
]. This suggests that MPs can
negatively impact the quality of the compost by affecting the formation of stable organic
matter. Moreover, the presence of MPs has been reported also to influence the microbial and
fungal communities within the compost by decreasing the diversity and richness of fungal
communities, particularly during the thermophilic stage of composting. For example,
the addition of PHA and PE MPs increased the relative abundance of phytopathogenic
fungi, leading to a simpler and more unstable fungal community structure [
87
]. Similarly,
the bacterial community’s richness and diversity were reduced in the presence of MPs,
with significant changes observed in the microbial community structure, especially in the
presence of polyvinyl MPs [
88
]. Given these aspects, it is no surprise that several studies
have identified the compost obtained from municipal solid waste as a potential source and
carrier of microplastics into the environment [
82
,
83
,
86
] emphasizing once more the need to
adopt improved waste management practices and source separation strategies to mitigate
environmental contamination [82,83].
Pharmaceuticals and personal care products (PPCPs) are a wide range of substances
including antibiotics, painkillers, hormones, and cosmetic ingredients that may occur
in the municipal solid waste stream used as feedstock in composting. Several studies
have investigated the occurrence and fate of PPCPs during composting processes and the
environmental impact of their application on soil as fertilizer.
Research has shown that certain PPCPs are degraded during the composting process
with varied efficiencies depending on the specific particularities of the chemical compound
and the composting process conditions. Thomas et al. (2020) emphasized that during
septage co-composting using an in-vessel technology the pharmaceutical carbamazepine
(CBZ) could be degraded with efficiencies up to 83% during single pollutant degradation,
while the personal care product triclosan (TCS) showed a removal rate of 86% under similar
conditions. Moreover, the study also indicated that the complexity of the waste matrix
can influence the degradation process as the presence of multiple pollutants reduced the
degradation efficiency to 66% for CBZ and 83% for TCS [
89
]. Another study showed that
composting significantly reduces levels of extractable antibiotics in livestock manure, with
calculated half-lives ranging from 0.9 to 16 days for most antibiotics [
90
]. In addition,
Mitchell et al. (2015) reported the degradation of antibiotic compounds from manure and
biosolids with efficiencies greater than 85% within 21 days of thermophilic composting [
91
].
However, even if antibiotic concentrations can be significantly reduced, the fate of antibiotic
resistance genes remains a complex issue that requires further consideration to fully include
environmental implications [90,92] (Table 2).
MSW is a mixture of different organic and inorganic materials that can contain a
variety of PTS that might threaten human health and the environment if not properly
handled. Composting is a promising alternative to managing organics in MSW; however,
it is important to know what happens to the PTS in this situation, thus supplementary
research is needed to mitigate the risks associated with PTS accumulation in compost and
the potential environmental impact.
Table 2. PTS in MSW fractions and potential sources in composting.
PTS Category Examples in MSW Potential Sources References
Heavy metals
Lead (Pb), Cadmium (Cd), Mercury
(Hg), Chromium (Cr), Arsenic (As),
Copper (Cu)
Batteries, electronics, paints,
certain food waste [2,17,75,77,79]
Sustainability 2024,16, 6329 10 of 25
Table 2. Cont.
PTS Category Examples in MSW Potential Sources References
Persistent Organic Pollutants
(POPs)
Polychlorinated Biphenyls (PCBs),
Dioxins, Flame Retardants
Treated wood, plastics, and
certain textiles [36,80]
Microplastics Microplastic fragments
<5 mm
Packaging and textiles
synthetic fibers, atmospheric
deposition
[24,82,83,85,93]
Pharmaceuticals and Personal
Care Products (PPCPs)
Antibiotics,
Hormones,
Medications
Improper disposal [89,90,92]
6. Composting Benefits and Challenges
Municipal organic waste composting offers significant environmental benefits over tra-
ditional landfill disposal such as a reduction in GHG emissions, landfill life extension, reduc-
tion of environmental pollution, production of valuable compost, and
resource conservation.
Literature data indicate that composting municipal organic waste can considerably
lower GHG emissions and enhance carbon sequestration. Composting organic waste is
widely known to help divert it from landfills, thereby diminishing methane (CH
4
) emissions,
a potent GHG, generally generated through anaerobic decomposition in landfills [
45
,
94
–
96
].
For instance, a case study performed for a university revealed that composting source-
separated organic waste could reduce net GHG emissions by up to 47% compared to
traditional landfilling [
95
]. Correspondingly, in Bangladesh, an integrated system of pyrol-
ysis and composting of municipal organic waste was reported to reduce GHG emissions by
503 CO
2
e t
−1
municipal waste annually [
97
]. In addition to contributing to GHG emissions
mitigation, composting also helps to reintegrate significant amounts of recycled nutrients
back into the soil, improving soil properties by increasing soil organic carbon (SOC), enhanc-
ing microbial activity, and improving water retention and nutrient availability [
10
,
98
,
99
].
This consequently leads to a decreased requirement for synthetic fertilizers, thus the GHG
emissions from their production and application are also reduced [
12
,
98
,
100
,
101
]. However,
it is worth mentioning the composting process can also produce GHGs like carbon dioxide
(CO
2
), methane (CH
4
), and nitrous oxide (N
2
O), but in comparison to landfilling the GHG
emissions are generally at low levels and can be mitigated to some extent by using effective
management practices of the composting process such as proper aeration [45,102,103].
Municipal organic waste composting is proven to significantly prolong landfill opera-
tional life because it reduces important waste volumes that otherwise would be landfilled.
A study focusing on composting food and garden waste highlighted the possibility of
reducing the volumetric waste disposal while keeping the same waste production and thus,
increasing the landfill life from half a year to 4 years depending on the scenario and year
considered [
104
]. In the same way, composting may presumably resolve the landfilling
issues poor countries have. Organic waste may be removed by composting between 40%
and 50% of total landfill waste in Ukraine contributing to the considerable mitigation of
waste accumulation in landfills, decreasing the ecological impact of landfilling, and en-
hancing environmental safety [
105
]. In addition, the degradation of solid waste by aerobic
methods within landfill sites, as documented in Georgia (USA), increases the rate of waste
degradation; thereby, extending the operational life of the landfill and reducing associated
methane gas and leachate production [106].
Organic waste composting is a very important process with high potential to serve
as a tool for managing different types of environmental pollution in sectors such as air
pollution, soil pollution, water pollution, etc., [
107
]. Separating biodegradable organic
waste from landfills suppresses methane emissions associated with uncontrolled anaerobic
degradation of organic matter. In addition, a properly managed composting system can
minimize the emission of other air pollutants including volatile organic compounds (VOCs)
and ammonia NH
3
[
108
,
109
]. At the same time, composting can also contribute to soil
Sustainability 2024,16, 6329 11 of 25
health by improving soil structure and nutrient profile or by enhancing soil microbial
activity which can help immobilize heavy metals and other contaminants, thus reducing
soil pollution [
110
–
112
]. Furthermore, by minimizing the production of leachate that is
normally generated in landfills, composting can also help tackle water pollution as leachate
could contaminate both groundwater and surface water [
113
,
114
]. In essence, municipal
organic waste composting is an eco-friendly treatment method not only for recycling
precious nutrients back into the soil but also for minimizing the significant environmental
effects caused by waste disposal [113,115].
The reduction of secondary pollutants through source separation of the organic frac-
tion and municipal organic waste composting is another environmental benefit. Source
separation of organic waste prevents contamination with heavy metals and other pollu-
tants, resulting in higher-quality compost. This method minimizes the need for extensive
waste pretreatment, thereby reducing the generation of bioaerosols and malodors during
the composting process [
2
]. Furthermore, it has also been shown that source-separated
composting reduces methane emissions and elution of ammonium nitrogen, both common
forms of contamination associated with mixed waste composting [
116
]. Ensuring that only
clean organic material enters the composting process minimizes the levels of carcinogens
and other potentially toxic contaminants occurring within the final compost product. By
reducing these hazardous materials, the public acceptance and marketability of the compost
itself are enhanced [
117
]. Furthermore, by collecting organic waste separately from specific
sources, such as restaurants operating in vegetable markets, there is no concern about toxic
materials, thus making the final product much safer, by not having to address secondary
pollution [118].
OFMSW composting is a sustainable waste management solution, but also a way
to enrich soils. As studies demonstrated, the composting process leads to a biofertilizer
with increased levels of essential elements for crops such as nitrogen (N), phosphorous (P),
and potassium (K) that can be used for soil amendment and
fertilization [16,119,120]
. For
instance, a study focusing on thermophile pre-composting of MSW for 3 weeks followed by
vermicomposting with the addition of cow dung has been found to enhance the decomposi-
tion and mineralization rates, resulting in a final compost with higher nutrient content [
119
].
Additionally, the integration of biological processes like anaerobic digestion and ozonation
can further improve the quality of the compost by reducing organic contaminants, thus
making it safer for soil application [
121
]. Field studies have demonstrated higher yields
for crops such as corn and rice and better condition of soil health when compost resulting
from MSW has been applied [16,122].
Composting of municipal organic waste also provides economic benefits for the com-
munity as it is a sustainable waste management method. The economic advantages of
composting include extended landfill life [
90
,
109
,
123
], reduced waste management cost, rev-
enue generation by selling compost, and mitigated emission of GHGs which can generate
capital by carbon credits [
97
,
124
,
125
]. Medium-scale to lower large-scale composting plants
are reported as the most economically viable option as they are capable of ensuring better
control of waste input, better process control, and consequently a high-quality end-product.
At these scales, there are further opportunities to earn money for closed-loop systems
and other income like tipping fees and carbon credits [
124
]. Therefore, composting can
create labor, develop local economies by the establishment of new industries for compost
production and sales, help the circular economy by closing the waste loop, and enhance
sustainable waste management practices [126,127].
Compost-amended soils have more diverse and stable microbial communities, and
this may improve the ability of these soils to withstand adverse conditions and promote
more sustainable agricultural practices [
128
]. In addition, composting using inorganic
bulking agents has been shown to enhance the nutrient profile of the compost, making it a
feasible component for potting soil or garden soil amendments [
10
]. Thus, utilization of
organic amendments from composting processes can result in the formation of sustainable
Sustainability 2024,16, 6329 12 of 25
soils by enhancing soil functionality and fertility through efficient recycling of organic
resources [129].
Composting municipal organic waste, although beneficial for soil amendment and
waste management, presents several challenges and potential drawbacks, including poten-
tial environmental problems (odors, bioaerosols, heavy metals, leachate, gas emissions),
operational difficulties (segregation, pathogen detection, composting duration, low levels
of adoption) and management constraints (toxic substances, lack of information, efficient
pricing systems and regulations). The high variability in the composition of organic waste is
a key issue that can have a large impact on the quality and consistency of the final composts
produced. For example, different types of organic waste may contain varying concentra-
tions of nutrients and heavy metals, which further affects the intensity of the composting
process, the quality of the final compost, and its suitability for reuse in
agriculture [130–132]
.
In addition, impurities and contaminants such as plastics and metals complicate the com-
posting process and lower the quality of composts in production [
133
,
134
]. The main
challenges regarding environmental impact are related to GHG (e.g., methane and nitrous
oxide) emissions, odors, and heavy metals management. Odor problems may occur dur-
ing the composting process as the result of volatile organic compounds (VOCs) such as
ethyl isovalerate and terpenes generation and release, leading to complaints and potential
facility closures [
75
,
135
]. Bioaerosols are another environmental issue related to bacteria
and fungi in the emissions of composting facilities or during compost application, which
may pose a potential health risk to workers, the local environment, and overall community
health [
136
,
137
]. These emissions not only contribute to climate change but also affect the
local environment and community health [
134
,
138
]. Finally, the management of heavy
metals and other pollutants in compost is absolute to prevent soil contamination and secure
compost’s safety for agriculture applications [
2
,
138
]. Efficient source separation of organic
waste is mandatory to prevent contamination (with heavy metals and other contaminants)
and improve the composting process efficiency [2].
The health risk related to potential pathogens occurring in compost derived from
municipal organic waste is another issue of concern. Studies show a varied degree of
pathogen reduction achieved depending on the composting method of choice (especially
delineating between composting and vermicomposting of OFMSW); several studies re-
ported that vermicomposting resulted in higher pathogenic bacteria reduction, including
E. coli
, compared to traditional composting, most probably due to all the complex digestive
processes in earthworm guts [139,140].
Moreover, bioaugmentation with specific microbial inoculants such as the white-rot
fungi proved to enhance the compost degradation intensity and maturation resulting
also in decreased pathogen loads [
141
]. However, the occurrence of pathogenic Candida
species in the early stage of composting demonstrated the necessity of thorough monitoring
and control measures [
142
]. Thus, further research is necessary to fully understand the
mechanisms of pathogen reduction in both composting and vermicomposting processes to
ensure the production of safe and high-quality compost [142,143].
At last, the economic and operational challenges for the broad adoption of composting
as a viable and sustainable waste management strategy are related to the high costs of
infrastructure and the need for continuous monitoring and optimization of the composting
process [144,145].
As summarized in Table 3, several benefits arise from the composting of municipal
organic waste: It diverts that material from landfills, and, as waste that is not put into
a landfill decomposes, it does not instead break down into methane. Composting also
recycles organic nutrients in the waste back into the soil—nearly closed-loop by some
models and beneficial to even the most sustainable agriculture practices. Nevertheless,
composting organic waste from municipalities is difficult due to its mixed constitution.
This is mainly due to the different recalcitrant elements of the wood–cellulose complex
in the overall waste composition, which can extend the composting time and reduce the
quality of the final product. The emissions of greenhouse gases and air pollutants that
Sustainability 2024,16, 6329 13 of 25
take place during composting are a downside to this seemingly eco-friendly process, for
which pile management and aeration are key aspects for mitigating the ineffective public
engaging strategies of this technology.
Table 3. Composting Municipal Organic Waste: Benefits and Challenges.
Aspect Benefit Challenge
Greenhouse Gas
Emissions (GHG)
Reduces methane emissions from landfills and
increases carbon sequestration in soil
Low-level GHG emissions during
composting
Landfill Life
Reduces waste volume sent to landfills thus extending
landfills lifespan
Pollution Reduction
Reduces air pollution (VOCs, ammonia)
Reduces soil pollution (heavy metals)
Reduces water pollution (leachate)
Odors from composting process
Management of heavy metals in
compost
Compost Production Creates nutrient-rich fertilizer for
soil amendment
Economic Benefits
Reduced waste management costs
Revenue generation from compost sales
Carbon credits for mitigated
GHG emissions
High infrastructure costs
Operational difficulties (monitoring,
optimization)
Low adoption rates
Soil Enhancement
Improves soil health (structure, water retention,
nutrients)
Promotes sustainable agriculture
Potential for pathogen presence in
compost
A comparison of municipal solid waste composting, vermicomposting, and larvae or
cricket production for organic waste management is presented in Table 4.
Table 4. Benchmarking of municipal organic waste composting, vermicomposting, and larvae or
cricket production.
Composting Vermicomposting Larvae or Cricket Production
Feedstock Mixed organic waste Food scraps and yard
trimmings
Organic waste streams
(including food scraps)
Process Decomposition by
microorganisms in piles or bins
Decomposition by worms in
bins or containers
Decomposition by insect larvae
in containers
End product Compost Vermicompost
Insect frass (manure) for animal
feed or fertilizer,
Insects as protein source for
animal feed
Advantages
Large-scale processing Smaller-scale processing
suitable for households Efficient waste conversion
Reduces landfill waste Low odor High-protein insect production
Creates soil amendment High-quality compost Reduces reliance on fishmeal
Reduced pathogen load
Disadvantages
Requires proper management to
avoid odor and methane
emissions
Lower processing rates than
composting
Requires careful management to
prevent escapes
Risk of pathogen contamination Requires specific feedstock for
worms Potential allergens
All three methods divert waste from landfills. A 2022 study by Nigussie et al. [
146
]
found vermicomposting to be particularly effective, potentially diverting up to 70% of
Sustainability 2024,16, 6329 14 of 25
organic waste. Composting produces a good soil amendment but may have lower nutrient
content than vermicompost. Studies showed vermicompost to have higher levels of avail-
able nitrogen and phosphorus [
9
,
67
]. Insect frass can also be a valuable fertilizer, but its
specific composition depends on the insect species and feedstock [147].
Throughput and processing time are important factors. Composting can handle large
volumes but has a longer processing time. Vermicomposting excels in smaller-scale settings
while having a higher time requirement. Insect production can be efficient, with studies
like that by Manaa et al. [
148
] in 2024 demonstrating high conversion rates of organic waste
into insect biomass.
Labor, equipment, and maintenance vary. Composting facilities often require signif-
icant infrastructure investment, while vermicomposting has lower operational costs but
limited scalability. Insect production can be cost-effective, with research by Li et al. [
149
] in
2023 suggesting potential economic benefits.
Odor, greenhouse gas (GHG) emissions, and leachate generation are critical sustain-
ability considerations. While all three methods can be managed to minimize odor, vermi-
composting generally has lower emissions. A 2021 study by Sayara et al. [
150
] highlights
the potential for composting to generate methane, a potent GHG. On the contrary, insect
production can also contribute to GHG reduction compared to traditional
protein sources.
Thus, the ideal organic waste management approach depends on specific circum-
stances. Composting excels in large-scale waste diversion, vermicomposting performs
better in smaller settings and low-odor applications, while insect production offers a
promising future for waste reduction and protein production.
7. Case Studies and Success Stories
Various case studies emphasize the potential of using municipal organic waste as a sus-
tainable waste management and nutrient recycling, while improving soil. Compost-derived
nutrients offer a sustainable solution for improving soil and fertility in various applications,
including soil modification, agricultural use and landscape. The process of composting
biodegradable organic waste not only recycles nutrients, but also contributes to improving
the productivity of crops and soil health by increasing the chemical, physical and biological
properties of soils [
34
,
119
,
151
]. In agriculture, it has been shown that the application of
compost deviates organic waste from landfills, thus reducing methane emissions [
130
].
The application of compost or vermicompost on soils has demonstrated positive effects on
tomato yield and nutritional status, indicating the potential of these organic changes to
replace mineral fertilizers in intense agriculture [
152
]. Furthermore, application of organic
amendments such as compost has been shown to influence soil microbial communities,
which play a critical role in soil health and nutrient cycling [
98
,
153
]. Composting municipal
waste with agricultural by-products can produce high-quality composts for agricultural
soils recovery, emphasizing the versatility of compost applications [
154
,
155
]. In landscaping
and soil remediation, MSW compost represents a potential renewable P source, improving
soil P status without posing significant environmental risks [
155
]. Following MSW compost
repeated application, soil C and N content has been enriched, enzymatic activities have
been modified and microbial community dynamics impacted, relevant to both agricultural
and landscaping uses [
45
]. Another study emphasized that application of garden waste
compost improves desalination efficiency, nutrient availability, and microbial diversity in
coastal saline soils, ensuring an ecological restoration practice [
129
]. In vineyards, compost
application has been recently considered because of its potential to improve soil health,
whilst considering the key environmental trade-off—GHGs emissions [156].
One of the major initiatives is composting the organic fraction of municipal solid
waste—to minimize landfill disposal, as has been emphasized by several
studies [104,106,126]
.
However, its adoption varies from country to country due to socio-demographic factors
and the need for greater awareness and social commitment to encourage its use [
157
].
Another innovative approach reported in the literature includes composting of MSW
with earthworms and ligno-cellulolytic microbial consortia for sustainable reclamation
Sustainability 2024,16, 6329 15 of 25
of degraded sodic soils, highlighting the potential of compost to improve soil health and
productivity [42,64].
The success of OFMSW composting programs relies on a combination of policy sup-
port, community engagement, and technological advancements. Additionally, policy
incentives play a crucial role in encouraging consumers to adopt environmentally friendly
disposal strategies, which aligns economic competitiveness with environmental objectives,
thereby incentivizing residents to minimize GHGs emissions through preferred disposal
patterns [158,159].
Key factors on the community engagement level are education and social interactions,
that have the most contribution to adopting sustainable practices. Using compost from
OFMSW can improve sustainability and research confirm that education level, peer or
social network suggesting and level of interest in adopting OFMSW compost correlated
significantly. This indicates the role of institutions and policymakers to facilitate the
knowledge and support the success of local initiatives in the future [13].
From the perspective of minimization of the environmental and health impacts as-
sociated with organic waste treatment, technological advancements have been made in
fields such as optimized composting processes and bioaerosols management in composting
plants. By advancing bioaerosol management, exposure levels were lowered and risk
assessments improved, thus enhancing the environmental performance of composting
facilities [
160
,
161
]. Additionally, applying life cycle assessment (LCA) methodologies to
evaluate the environmental impacts of waste management facilities, including the poten-
tial for renewable energy solutions, demonstrates the benefits of these advancements in
reducing environmental impacts and improving energy efficiency [
132
]. Furthermore, the
development of technologies for the composting of MSW using earthworms and microbial
consortia showcases the potential for enhancing the nutrient content of compost [
162
,
163
].
Finally, research studies on nutrients and potentially toxic elements contents emphasize the
importance of monitoring and managing the compost’s quality to ensure its safe application
on soils [163].
The impact of legislation on landfilled waste and composting is clear in waste disposal
European statistics (Figure 2). The EU Landfill directive implementation resulted in a
continuous decrease in landfilled waste volumes, as well as in an acceleration in other
processing approaches. In the last twenty years, an increase of over 200% in the composted
waste quantities has been identified, contributing to a considerable reduction in landfilled
waste reduction.
Sustainability 2024, 16, x FOR PEER REVIEW 16 of 26
Figure 2. Municipal waste treatment in Europe (1995–2022). Source: Eurostat.
Another key change in waste management approach in Europe was the implemen-
tation of the European Commission’s Circular economy package, that emphasized the im-
portance of resources recovery thus accelerating the implementation of treatment meth-
ods other than landfilling.
Local initiatives also have significant contributions on separate organic waste collec-
tion and composting. Two examples are the Mandatory Organics Recycling Ordinance
(2009) form San Francisco, California that envisages separate organics collection from all
residents and businesses and Organics Recycling Law (2014) in Maine that offers technical
assistance to food waste generators to divert it to composting instead of landfilling. Both
initiatives led to decreased landfilled waste volumes and increased compost production.
8. Discussions and Future Directions
The demand for improved, greener, and certainly scalable composting solutions is
even more important considering growing cities, and higher quantities of waste being
generated. Serious opportunities for coping with the ever-mounting problems of waste
management and the imperatives of resource sustainability are presented in the field of
intensive composting of organic urban waste. This chapter explores future directions and
current research needs in order to advance the field of intensive municipal organic waste.
Future research and innovation in technological progress, microbial and biochemical per-
spectives, process optimization, environmental and economic evaluation, as well as polit-
ical and public involvement are essential to unlock the full potential of composting sys-
tems.
Given the great importance of the quality and homogeneity of the organic waste feed-
stock for the efficiency of composting processes and for the predictability of the final com-
post quality, it is necessary to identify future research actions aiming to develop advanced
technologies for organic waste separation and pre-treatment with proper efficiency and
minimum monetary and environmental costs. The study on the viability of the used tech-
nologies is proposed by the development of macro-screening inspection assemblies inte-
grated with sorting and inspection of waste streams, in terms of organic and inorganic
waste contamination. Automated sorting and current separating systems, supported by
artificial intelligence (AI) systems and by machine learning (ML) techniques, will allow
the enhancement of sorting accuracy and efficiency.
Because the efficiency of the composting process and the quality of compost are in-
fluenced by the main operational parameters, it is critically important to maintain proper
aeration, humidity, and temperature in order to achieve an efficient composting process
[157]. Thanks to the advancements of sensor technologies and Internet of Things (IoT), it
0
50
100
150
200
250
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
Million tons of waste
Landfill Composting Total
European Commission's Circular Economy Package
EU Landfill Directive
Figure 2. Municipal waste treatment in Europe (1995–2022). Source: Eurostat.
Sustainability 2024,16, 6329 16 of 25
Another key change in waste management approach in Europe was the implemen-
tation of the European Commission’s Circular economy package, that emphasized the
importance of resources recovery thus accelerating the implementation of treatment meth-
ods other than landfilling.
Local initiatives also have significant contributions on separate organic waste collection
and composting. Two examples are the Mandatory Organics Recycling Ordinance (2009)
form San Francisco, California that envisages separate organics collection from all residents
and businesses and Organics Recycling Law (2014) in Maine that offers technical assistance
to food waste generators to divert it to composting instead of landfilling. Both initiatives
led to decreased landfilled waste volumes and increased compost production.
8. Discussions and Future Directions
The demand for improved, greener, and certainly scalable composting solutions is
even more important considering growing cities, and higher quantities of waste being
generated. Serious opportunities for coping with the ever-mounting problems of waste
management and the imperatives of resource sustainability are presented in the field of
intensive composting of organic urban waste. This chapter explores future directions and
current research needs in order to advance the field of intensive municipal organic waste.
Future research and innovation in technological progress, microbial and biochemical per-
spectives, process optimization, environmental and economic evaluation, as well as political
and public involvement are essential to unlock the full potential of composting systems.
Given the great importance of the quality and homogeneity of the organic waste
feedstock for the efficiency of composting processes and for the predictability of the final
compost quality, it is necessary to identify future research actions aiming to develop ad-
vanced technologies for organic waste separation and pre-treatment with proper efficiency
and minimum monetary and environmental costs. The study on the viability of the used
technologies is proposed by the development of macro-screening inspection assemblies
integrated with sorting and inspection of waste streams, in terms of organic and inorganic
waste contamination. Automated sorting and current separating systems, supported by
artificial intelligence (AI) systems and by machine learning (ML) techniques, will allow the
enhancement of sorting accuracy and efficiency.
Because the efficiency of the composting process and the quality of compost are influ-
enced by the main operational parameters, it is critically important to maintain proper aer-
ation, humidity, and temperature in order to achieve an efficient composting process [
157
].
Thanks to the advancements of sensor technologies and Internet of Things (IoT), it is becom-
ing possible to monitor and control composting parameters in real time. Future research
should be focused on developing intelligent composting systems to automatically adjust
aeration and temperature to optimize the microbial activity and accelerate composting. In
this field of research, continuous thermophilic composting has become more important
because it has the potential to reduce processing time and improve compost stability, thus
becoming more attractive for commercial applications [164].
Moreover, improved control and efficiency of the composting process and tailored
nutrient transformations can be achieved by a deeper understanding of microbial com-
munity dynamics. Identification of key microbial species and their interactions towards
optimized degradation of organic wastes and improved physico-chemical dynamics during
composting should also be a focus of future research [
10
,
23
]. Dedicated metagenomic and
metatranscriptomic approaches can provide insights into microbial functions and the path-
ways involved in composting. Integrated metagenomic and metatranscriptomic analyses
could offer the means to understand microbial gene composition and expression in complex
environments, such as the composting systems. Thus, these combined approaches might
elucidate the functional roles of microbial communities, identify key metabolic pathways
involved in organic matter degradation, and provide insights into the dynamic interactions
within the microbial ecosystem [165,166].
Sustainability 2024,16, 6329 17 of 25
In addition, researchers might explore whether certain enzymes can be added to
composting systems to hasten the decomposition of complex organic compounds [
167
–
169
].
Additionally, genetic engineering techniques could be employed to enhance the activity
and stability of these enzymes under composting conditions.
The quality of the final compost product is critical for acceptance and use as amend-
ment or fertilizer. Future studies should be focused on technologies to improve the compost
quality including optimal feedstock ratio, duration of composting, maturation processes.
In this regard, researches on the impact of different waste compositions on the nutrient
value, pathogen reduction, potential additives, presence of beneficial microorganism in
finished compost, should be investigated. For example, Gaspar et al. (2022) showed that
the phosphorus bioavailable content and the overall quality of the compost is improved
when specific microbial inoculants are added to the process [
39
]. In addition, the potential
of the biochar, especially oxidized biochar, to improve nitrogen retention during com-
posting has an innovative research path, which suggests the need to deepen more in the
physiochemical interactions between BioChar and Compost materials [
98
,
170
]. Moreover,
detailed studies on the selection and optimization of compost amendments would allow to
improve nutrient retention and organic matter degradation, including its effect on microbial
community structure and metabolic function [
10
]. Another area of research is to investigate
the occurrence and fate of contaminants and their potential environmental impact, as well
as the mechanisms responsible for their removal during composting [127].
In order to reach full sustainability, the progress of the composting process should
target also the sustainable use of energy and water resources. Thus, research should inves-
tigate the use of alternative energy sources, such as solar energy or bioenergy, to power
composting installations while different methods for minimizing water use should be de-
veloped or strategies to reuse process water should be implemented. In order for intensive
composting systems to be adopted at a wide-scale, it is important that their economic
viability is assessed. The research should look into cost-effective composting technologies
and business models that can be scaled to different installations. In addition, attempts
should be made to develop markets for compost products, this includes establishing qual-
ity standards and certifications to build consumer trust and demand [
171
–
173
]. Future
research could also focus on developing standardized LCA methodologies allowing to
benchmark various composting technologies and practices. The assessments will provide
valuable information on the overall sustainability of composting systems and could lead to
knowledge-based solutions for process improvement [171].
Effective regulatory frameworks are needed in order to facilitate the expansion of
intensive composting of municipal organic waste. To define the future research agenda,
existing regulations should be examined and potential gaps or obstructions to the imple-
mentation of composting projects should be noted. Actions and incentives that encourage
composting should be prepared to address the results obtained from this research.
In order for city composting programs to be successful, public engagement is manda-
tory. Research should also focus on identifying optimal opinion campaigns to teach and
grow public support for composting and recycling of organic waste. Public involvement
events can allow cities to explain how composting works and what they’re trying to
accomplish in countywide composting programs.
9. Conclusions
Composting municipal organic waste is an essential part of sustainable waste manage-
ment and agricultural practices as it is a valuable method of recycling nutrients. Far from
being a mere disposal method, composting contributes positively to soil health and food
production, and therefore to environmental sustainability and food security. Composting
offers substantial environmental benefits, including extending the life of landfills, reducing
GHGs emissions by diverting organic waste from landfills, and improving soil quality and
structure by using the compost produced as fertilizer. This practice could contribute to the
transition to a circular economy by closing the nutrient loop, decreasing dependence on fi-
Sustainability 2024,16, 6329 18 of 25
nite resources, and encouraging resource conservation and resilience. Although, apparently,
composting is a simple process and takes place in various conditions, careful management
of the process and control of minimum parameters such as temperature, humidity, oxygen
concentration and the C/N ratio would ensure the successful decomposition of municipal
organic waste in a valuable final product, rich in nutrients. These critical parameters govern
both the progress of the composting process and the potentially associated processes that
lead to the production of GHGs. Parameters control ensures proper waste degradation and
good quality compost, which, in turn, encourages correct waste management and use of
the resulting compost. Despite its benefits, composting faces challenges such as nutrient
leaching and potential heavy metal contamination. Proper management practices and
monitoring are required to meet these challenges and to guarantee the quality and safety of
compost products. Successful nutrient recycling initiatives rely also on strong policy frame-
works, community engagement and infrastructure support. In the future, an effort needs
to be made to fill research gaps, develop more effective and efficient composting systems,
establish alternative recovery pathways, and scale composting to the facilities necessary for
this platform to achieve extensive, impactful use as a circular economy tool. Scaling globally
would have far-reaching implications in terms of food waste reduction, soil restoration,
climate change mitigation, and leaping to a more sustainable and resilient future.
Author Contributions: Conceptualization E.E.M. and C.B.; formal analysis, M.B., C.M.N. and
L.R.D.; investigation, L.R.D., C.B. and E.E.M.;
writing—original
draft preparation, all authors;
writing—review
and editing, E.E.M., C.B. and C.M.N.; supervision, C.B. and E.E.M.; project ad-
ministration, C.B. All authors have read and agreed to the published version of the manuscript.
Funding: This work was carried out through the “Nucleu” Program within the National Research
Development and Innovation Plan 2022–2027 with the support of Romanian Ministry of Research,
Innovation and Digitalization, contract no. 3N/2022, Project code PN 23 22 03 02.
Conflicts of Interest: The authors declare no conflicts of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or
in the decision to publish the results.
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